1. Field of the Invention
The present invention relates to a magnetron utilized for a microwave oven and the like, and more particularly to a magnetron having improved choke means for harmonic suppression.
2. Description of the Background Art
FIG. 1 is a view schematically showing the structure of a microwave oven utilizing a magnetron. Referring to FIG. 1, a microwave oven 1000 has a magnetron 100, a driving power supply 200 for driving the magnetron 100, and a waveguide 300. The microwave oven 1000 is entirely covered with a microwave oven cover 400. Microwaves produced by the magnetron 100 are guided into a space 500 in the microwave oven through the waveguide 300. Microwaves so guided heat and cook food 700 placed on a plate 600.
FIG. 2A is a partially sectional front view showing the structure of a conventional magnetron. FIG. 2B is a partial sectional view taken along line IIB--IIB in FIG. 2A. FIG. 2C is a partially sectional view taken along line IIC--IIC in FIG. 2B. Referring to FIGS. 2A-2C, a typical structure of a conventional magnetron will be described.
Referring to FIGS. 2A-2C, a cathode 3 is disposed in the center of a magnetron 100. The cathode 3 has a filament 5 (see FIG. 2C) and emits electrons. A plurality of plate-shaped vanes 2 of oxygen-free copper or the like are disposed radially to encircle the cathode 3. The vanes 2 have the base ends fixed to the inner wall of an anode cylinder 1 formed of oxygen-free copper or the like, or formed integrally with the anode cylinder 1.
Two inner strap rings 9 selected to be identical in diameter are provided at the upper and lower ends of the vanes (in FIGS. 2A and 2C). The inner strap rings 9 are disposed at a prescribed distance from the tip ends of the vanes 2 with respect to the entire length of the vanes 2. Two outer strap rings 10 selected to be identical to each other in diameter and larger in diameter than the inner strap rings 9 are provided at the upper and lower ends of the vanes 2. The inner strap rings 9 and the outer strap rings 10 are fixed to the vanes 2 to short-circuit every other vane 2. In other words, the upper inner strap ring 9 and the lower outer strap ring 10 are fixed to the same alternately disposed vanes 2, and the upper outer strap ring 10 and the lower inner strap rings 9 are fixed to the remaining vanes 2, respectively.
Two adjacent vanes 2 and the inner wall of the anode cylinder 1 surround spaces 14 (see FIG. 2B) partially opened toward the cathode 3 thereby forming cavity resonators. The oscillation frequency of the magnetron 100 is determined depending upon the resonant frequency of the cavity resonators. In the center of the anode cylinder 1, a cylindrical space is axially defined by the tip ends of the vanes 2. The cathode 3 is arranged in the space. As seen in FIG. 2B, a space 4 is formed between the cathode 3 and the vanes 2 at a prescribed distance is called an interaction space. A uniform direct-current magnetic field is applied to the interaction space in parallel with the central axis of the cathode 3. For this purpose, permanent magnets 12 (see FIG. 2A) are arranged in the vicinity of the upper and lower ends of the anode cylinder 1, respectively. A direct-current or low-frequency high voltage is applied between the cathode 3 and the vanes 2.
As seen in FIG. 2C, the cathode 3 is formed by the filament 5 fabricated helically from tungsten containing thorium and the like, a top hat 7 supporting the upper end of the filament 5 and having a flange 6 which is larger in outer diameter than the filament 5 at the top, and an end hat 8 supporting the lower end of the filament 5. The top hat 7 and the end hat 8 are formed of refractory metal such as molybdenum. The top hat 7 and the end hat 8 prevent electrons from deviating axially from the filament 5.
Alternate ones of the vanes 2 are electrically connected with each other, since the inner strap rings 9 and the outer strap rings 10 are alternately fixed to the upper and lower ends of the vanes 2 as described above. An antenna conductor 11 (see FIGS. 2A, 2F) has one end connected to one of the vanes 2.
In the above mentioned structure, high frequency fields formed in the cavity resonators concentrate on the tip ends of the respective vanes 2 and leak in part into the interaction space 4. The adjacent vanes 2 have potentials reverse to each other at high frequency, since the inner and outer strap rings 9 and 10 couple alternate ones of the vanes 2. An electron group emitted from the cathode 3 spins about the cathode 3 in the interaction space 4 causing interaction between the electron group and the high frequency electric fields, and microwaves are produced as a result. The assembly is completed by an output side insulating tube 16, an exhaust pipe or tubulation 17 and an antenna cap 18.
The microwaves are guided outwardly through the antenna conductor 11 connected to one of the vanes 2. The energy of the electron group is partially consumed as heat, since the conversion efficiency into microwave power is not 100%. Therefore, fins 13 (see FIG. 2A) are provided for heat radiation along the outer circumference of the anode cylinder 1. FIG. 2B shows only the internal structure of the anode cylinder 1, and fins 13 etc. are not shown in the figure.
International standards are established by ITU (International Telecommunication Union) for a magnetron as mentioned above, and a basic frequency of 2,450 MHz is allocated to food heating apparatuses, medical instruments, some industrial instruments and the like. A magnetron used for the above mentioned apparatus and instruments ideally oscillates only microwaves at a fundamental frequency of 2,450 MHz (.+-.50 MHz), but in practice also generates various higher harmonics.
Among the microwave frequencies actually oscillated from a magnetron include various higher harmonics such as the second harmonic, the third harmonic and the like, and components other than the above range are also included in basic waves. When such a harmonic is propagated into the cavity of a microwave oven for example, the shorter the wavelength of the harmonic becomes, the harder will be the shielding thereof, resulting in the more outward leakage. Even a very weak leaky-wave of this kind can cause radio interference. Among such higher harmonics, the fifth harmonic having a frequency of 12.25 GHz (.+-.0.25 GHz) overlaps the working frequency range of satellite broadcasting which has been tested since around 1981 and recently put into practice. Though radiowave frequency allocation for SHF satellite broadcasting varies from nation to nation, the frequency band is set to be in a range of 11.7 to 12.5 GHz.
A technique has been conventionally known, which suppresses the radiation of radio waves having undesirable bandwidths by providing a 1/4 wavelength choke at the output of a magnetron itself. Such techniques are disclosed in Japanese Patent Publication No. 54-6862 (1979), Japanese Patent Laying-Open No. 61-288347 (1986), U.S. Pat. No. 4,833,367, etc. A magnetron provided with a choke has a structure as schematically shown in FIG. 3 which is a partially sectional view showing the upper end of an antenna conductor in the magnetron shown in FIG. 2A.
Referring to FIG. 3, a metallic container 15 and an output-side insulating tube 16 surround an antenna conductor 11 and define an airtight space inside the conductor. An exhaust pipe 17 and an antenna cap 18 are secured onto the output-side insulating tube. A choke body 19 is provided to surround the antenna conductor 11 in the metallic container 15. The length d of the groove of the annular groove-type choke body 19 is set to be approximately 1/4 wavelength of a harmonic, whose unwanted bandwidth emission is to be suppressed. Unwanted bandwidth emission corresponding to a prescribed harmonic can be thus suppressed by the choke body 19. Also by changing the length d of the groove appropriately, an arbitrary higher harmonic can be suppressed.
Although the conventional technique of suppressing unwanted bandwidth emission, as shown in FIG. 3, is effective in suppressing a harmonic of a relatively long wavelength such as the second harmonic and the third harmonic etc., sufficient effect cannot be obtained in suppressing a harmonic of a short wavelength such as the fifth harmonic of a microwave oven magnetron, which approximately coincides with the frequency band of 12 GHz for SHF satellite broadcasting. This is because the length l of the output-side insulating tube 16 shown in FIG. 3 should be about 10 mm in general due to the high-frequency insulating characteristic, and it is assumed that the inner space of the output-side insulating tube 16 is merely a space through which a harmonic of a short wavelength is radiated outwardly.
Furthermore, another problem related to the conventional technique is that, as shown in FIG. 4, a convex portion 152 can be formed by brazing material having flowed out in a brazed part 151 between the metallic container 15 and the output-side insulating tube 16. Electric field concentration due to a large microwave voltage can be caused between the convex portion 152 and the antenna conductor 11. The electric field concentration permits discharging between the convex portion 152 and the antenna conductor 11 thereby causing cracks in the output-side insulating tube 16 or gas to be released by locally heating the output-side insulating tube 16.
For solving the above mentioned problem, a structure is suggested in Japanese Utility Model Publication No. 58-910 (1983), in which an end to be brazed to the output-side insulating tube of a metallic container protrudes inwardly further than the inner diameter of the output-side insulating tube and is bent toward the output-side. However, the structure only eases the above mentioned electric field concentration and fails to suppress a harmonic having a short wavelength such as the fifth harmonic.